U.S. patent number 5,616,909 [Application Number 08/469,478] was granted by the patent office on 1997-04-01 for method and apparatus for maintaining a scanning optical path length within a predetermined range.
This patent grant is currently assigned to Intermec Corporation. Invention is credited to Kevork G. Arackellian.
United States Patent |
5,616,909 |
Arackellian |
April 1, 1997 |
Method and apparatus for maintaining a scanning optical path length
within a predetermined range
Abstract
A method and apparatus for maintaining a constant optical path
length when scanning objects at different distances. The apparatus
has a reading device that reads data carrying symbology on an
object being scanned, and an optical system having a plurality of
optical paths between the object being scanned and the reading
device, where each of the plurality of optical paths has a
different path length. The apparatus also has a size detector that
determines a dimension of the size of the object being scanned and
outputs a signal based on the dimension, and a selecting device
that selects one of the plurality of optical paths of the optical
system based on the signal output by the size detector to maintain
an optical path length between the object being scanned and the
reading device within a predetermined range.
Inventors: |
Arackellian; Kevork G.
(Everett, WA) |
Assignee: |
Intermec Corporation (Everett,
WA)
|
Family
ID: |
23863952 |
Appl.
No.: |
08/469,478 |
Filed: |
June 6, 1995 |
Current U.S.
Class: |
235/462.22;
235/462.2; 235/470 |
Current CPC
Class: |
G06K
7/10594 (20130101); G06K 7/10811 (20130101); G06K
7/10861 (20130101); G06K 2207/1013 (20130101) |
Current International
Class: |
G06K
7/10 (20060101); G06K 007/10 () |
Field of
Search: |
;235/472,467,470 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Pitts; Harold
Attorney, Agent or Firm: Seed and Berry LLP
Claims
I claim:
1. An optical apparatus for reading data present on different size
objects, said apparatus comprising:
a reading device that reads only a single image of the data on an
object positioned on a reference surface;
an optical system including a lens having a depth of field and a
plurality of optical paths each having a unique object plane,
wherein the object plane of each of the plurality of optical paths
is positioned at a different distance from a point on the reference
surface and wherein the object plane of each of the plurality of
optical paths is separated from an adjacent object plane of one of
the plurality of optical paths by a distance no greater than the
depth of field of the lens;
a size detector that determines the distance of the object from the
point on the reference surface; and
a selecting device that selects only one of the plurality of
optical paths with the object plane that has a distance from the
point on the reference surface closest to the distance from the
point of the object, and wherein the object falls within the depth
of field of the lens and the reading device substantially
simultaneously receives the single image of the data.
2. The optical apparatus according to claim 1 wherein said optical
system further comprises:
a redirecting element that redirects light from the object; and
a plurality of optical elements, each of which redirects light from
said redirecting element along a distinct field of view of the
lens, each of the plurality of optical elements being positioned a
predetermined distance from said redirecting element, and each of
the plurality of optical elements corresponding to one of the
plurality of optical paths.
3. The optical apparatus according to claim 2 wherein said
selecting device comprises:
a selector that selects the one of the plurality of optical paths;
and
driving device that moves said reading device along an image plane
of the lens to the distinct field of view corresponding to the one
of the plurality of optical paths that is selected.
4. An optical apparatus for reading data present on objects of
different height, said apparatus comprising:
a reading device that reads only a single image of the data on an
object;
an optical system having a plurality of optical paths between a
reference position and said reading device, wherein each of the
plurality of optical paths has a different optical path length to
the reference position;
a size detector that determines a distance of the data on the
object from the reference position and produces a signal based on
the distance of the data on the object from the reference position;
and
a selecting device that selects only one of the plurality of
optical paths of said optical system based on the signal produced
by said size detector to substantially simultaneously provide to
the reading device the single image of the data and to maintain an
optical path length between the object and said reading device that
is within a predetermined range.
5. The optical apparatus according to claim 4 wherein said optical
system further comprises:
a redirecting mirror that redirects light from the object;
a plurality of mirrors, each of which is positioned a predetermined
distance from said redirecting mirror, and each of which redirects
light from said redirecting mirror causing the light from each of
said plurality of mirrors to travel along one of the plurality of
optical paths; and
a lens that receives the light traveling along each of the
plurality of optical paths from said plurality of mirrors and
projects the light from each of the plurality of optical paths at
different positions along an image plane.
6. The optical apparatus according to claim 5 wherein said
selecting device comprises:
an electronic selector that selects the one of the plurality of
optical paths that will maintain the optical path length within the
predetermined range based on the signal produced by said size
detector, and produces a driving signal; and
a driving device that moves the reading device along the image
plane to a selected position in response to the driving signal.
7. The optical apparatus according to claim 6 wherein said driving
device further comprises:
a nut coupled to said reading device for movement therewith;
a threaded shaft engaged with said nut;
a slider coupled to said nut;
a slider guide and base assembly, said slider guide slidingly
engaged with said slider; and
a motor rotatingly coupled to said threaded shaft, said motor in
conjunction with slider guide moving the reading device to the
selected position in response to the driving signal.
8. The optical apparatus according to claim 6 wherein said driving
device comprises:
a shaft coupled to said reading device;
a flexure upon which said shaft pivots; and
a solenoid that causes said shaft to pivot about said flexure to
move said reading device to the selected position in response to
the driving signal and a position sensor.
9. The optical apparatus according to claim 5 wherein said reading
device comprises a CCD imager.
10. The optical apparatus according to claim 5 wherein said lens
comprises a fixed focal length lens.
11. The optical apparatus according to claim 9 for use with a
conveyor with a surface on which the object is transported wherein
the reference position is located at about the conveyor
surface.
12. The optical apparatus according to claim 11 wherein an object
plane of one of the plurality of optical paths having the least
height above the conveyor surface is set above the conveyor surface
by a minimum object height plus one-half the depth of field of said
reading device and lens.
13. The optical apparatus according to claim 12 wherein the depth
of field for said reading device and lens is approximately
determined by the f-stop of the lens, the pixel pitch of the CCD
imager and the object magnification of the lens.
14. The optical apparatus according to claim 13 wherein an object
plane of one of the plurality of optical paths having the greatest
height above the conveyor surface is set above the conveyor surface
by a maximum object height less one-half the depth of field of said
reading device and lens.
15. The optical apparatus according to claim 14 wherein the amount
of said plurality of mirrors is at least equal to the maximum
object height less the minimum object height divided by the depth
of field of said reading device and lens.
16. The optical apparatus according to claim 4 for use with a
conveyor with a surface on which the object is transported wherein
the reference position is located at about the conveyor
surface.
17. The optical apparatus according to claim 4 wherein said optical
system further comprises:
a plurality of mirrors, each associated with one of the plurality
of optical paths, each of said mirrors being positioned a
predetermined distance from each other of said plurality of mirrors
such that the optical path length associated with each of said
plurality of mirrors differs from the others of said plurality of
mirrors; and
a lens which projects light onto the reading device.
18. The optical apparatus according to claim 17 wherein said
selecting device controls an acousto-optic modulator that selects
one of said plurality of optical paths by redirecting light from
one of the plurality of mirrors to said reading device.
19. The optical apparatus according to claim 17 wherein said lens
comprises a fixed focal length lens.
20. The optical apparatus according to claim 17 wherein said
selecting device controls a selecting member that selects the one
of said plurality of optical paths by redirecting light from one of
said plurality of mirrors to said lens.
21. The optical apparatus according to claim 20 wherein said
selecting member further comprises a reflecting surface which is
rotatable to redirect the light from the one of said plurality of
mirrors to said lens.
22. The optical apparatus according to claim 20 wherein said
selecting member further comprises a reflecting surface which is
translated to redirect the light from the one of said plurality of
mirrors to said lens.
23. The optical apparatus according to claim 20 wherein said
selecting member further comprises a refractive element which is
rotated to redirect the light from the one of said plurality of
mirrors to said lens.
24. The optical apparatus according to claim 20 wherein said
selecting member further comprises a diffractive element which is
rotated to redirect the light from the one of said plurality of
mirrors to said lens.
25. The optical apparatus according to claim 20 wherein said
selecting member further comprises a diffractive element which is
translated to redirect the light from the one of said plurality of
mirrors to said lens.
26. A method for reading data on objects having different sizes,
said method comprising the steps of:
determining the size of an object with data to be read;
positioning a plurality of optical paths between the object and a
reading device; selecting only a single optical path from one of
the plurality of optical paths based on the size of the object,
and
reading only a single image of the data on the object via the
selected optical path.
27. The method of reading data according to claim 26 wherein each
of the optical paths has a different length.
28. A method of maintaining a constant optical path length range
when scanning objects with a data reading device at varying
distances, said method comprising the steps of:
receiving light from one of the objects with a redirecting
mirror;
redirecting the light from the redirecting mirror to a plurality of
mirrors;
positioning the plurality of mirrors at different distances from
the redirecting mirror;
producing a plurality of optical paths with the plurality of
mirrors;
projecting each of the optical paths onto an image plane with each
of the optical paths having a different field of view;
determining a height of the object being read; and
selecting only one of one of the fields of view based on the height
of the object being read.
29. The method according to claim 28, wherein the step of selecting
one of the fields of view includes the step of:
moving the data reading device to a position on the image plane to
acquire the field of view that will maintain the constant optical
path length range.
Description
TECHNICAL FIELD
The present invention relates to a method and apparatus for
scanning objects, and more particularly, to a method and apparatus
which maintains an optical path length within a predetermined range
when scanning objects at varying distances from a scanning
system.
BACKGROUND OF THE INVENTION
A prior an scanning device 108, as shown in FIG. 1, is typically
used to read symbology 110, such as a bar code, which consists of
alternating areas ("bars") having different reflectance/absorption
characteristics. The scanning device 108 receives the reflected
light and interprets the fluctuations in radiant emittance caused
by the reflectivity characteristics of the symbology 110. When the
scanning device 108 is held in a stationary position, the object
beating the bar code symbology 110 must be brought within the
working range of the scanning device 108. This can be done with a
conveyor belt 123. In "over-the-belt" scanning applications where
objects 150 of different sizes travel underneath the scanning
device 108, the distance from the scanning device 108 to an object
114 being scanned will change based on the size (height) of the
object 114. When the optical path length 120 between the scanned
object 114 and the scanning device 108 changes significantly, a
number of problems will be encountered. The most obvious problem is
loss of focus. This can be corrected with an automatic focusing
mechanism built into a scanning lens system 112. The less obvious
problem is that of changes in magnification. To compensate for such
changes one must use a variable focus (zoom) system. The most
practical and effective zoom system is based on changing the focal
length of the scanning lens system 112. To achieve changes in focal
length one or more lens groups must be moved within the scanning
lens system 112.
Typically, zoom lens systems for industrial applications will be
custom made when image quality and long life span are critical.
This means that such variable focal length lenses are more
expensive. Due to the need for the scanning lens system 112 to move
lens group(s) for zooming and focusing, the optical performance of
the system will be lower. Such zoom lenses typically have a lower
modulation transfer function (MTF) than a fixed focal length lens,
where MTF describes the modulation (contrast) of the image as a
function of the spatial frequency of the object. Moving lenses will
also place strict requirements on the actuators, which must move
lenses with high precision very fast through millions of cycles.
Any tilt or sideways shift introduced through the motion of the
moved lenses will further degrade the image quality. Accordingly,
"off-the-shelf," inexpensive lenses cannot be used in the scanning
lens system 112 when the optical path length 120 significantly
varies outside of the depth of field of the scanning lens system
112 while reading a sequence of symbologies 110 on different size
objects 150. In the past, one was restricted to either using
multiple scanners or lower performance but expensive custom lens
systems.
SUMMARY OF THE INVENTION
It is an object of the present invention to eliminate the need both
for focusing and a variable focal length lens to compensate for a
change in the optical path length of a scanning system.
It is another object of the present invention to provide a method
and apparatus for scanning that uses an optical system which is
less expensive and more robust and durable in mechanical design by
requiring fewer moving components and minimizing component
travel.
It is still another object of the present invention to provide a
method and apparatus for scanning that uses an optical system which
requires less power by having moving components that are low in
weight and fast in response time.
It is a further object of the present invention to provide a method
and apparatus for scanning targets at variable distances from a
scanner that uses an optical system having a fixed focal length
lens that is inexpensive and easy to obtain and that can be
purchased "off the shelf."
It is yet another object of the present invention to use components
which will significantly increase the life of the scanning
device.
In one aspect, the invention is a scanning apparatus for scanning
different sized objects (i.e., different distances from the
scanner). The apparatus comprises a reading device that reads or
scans data on an object positioned on a reference surface and an
optical system including a lens having a depth of field and a
plurality of optical paths each having a unique object plane,
wherein the object plane of each of the plurality of the optical
paths is positioned at a different distance from a point on the
reference surface, and wherein the object plane of each of the
plurality of optical paths is separated from an adjacent object
plane from one of the plurality of optical paths by a distance no
greater than the depth of field of the lens. The apparatus also
comprises a size or distance detector that determines the distance
of the object from the point on the reference surface, and a
selecting device that selects one of the plurality of optical paths
with the object plane that has a distance from the point on the
reference surface closest to the distance from the point of the
object being scanned, whereby the object being scanned falls within
the depth of field of the lens.
In another aspect, the invention is a scanning apparatus for
scanning objects of different height. The apparatus comprises a
reading device that reads data on an object being scanned, and an
optical system having a plurality of optical paths between a
reference position and the reading device, wherein each of the
plurality of optical paths has a different optical path length to
the reference position. The apparatus also comprises a size
detector that determines a distance of the data on the object from
the reference position and outputs a signal based on the distance
of the data on the object from the reference position, and a
selecting device that selects one of the plurality of optical paths
of the optical system based on the single output by the size
detector to maintain an optical path length between the object
being scanned and the reading device that is within a predetermined
range.
In another aspect, the invention is a method for scanning objects
having different heights. The method comprises the steps of
determining the size of an object being scanned, positioning a
plurality of optical paths between the object being scanned and a
reading device, selecting an optical path from one of the plurality
of optical paths based on the size of the object being scanned, and
reading data on the object being scanned with the reading device
via the selected optical path.
In yet another aspect, the invention is a method of maintaining a
constant optical path length range when scanning objects with a
scanning device at different distances. The method includes the
steps of receiving light from one of the objects being scanned with
a redirecting mirror and redirecting the light to a plurality of
mirrors, positioning the plurality of mirrors at different
distances from the redirecting mirror, producing a plurality of
optical paths with the plurality of mirrors, projecting each of the
optical paths onto an image plane, wherein each of the optical
paths has a different field of view, determining a height of the
object being scanned, and selecting one of the fields of view based
on the height of the object being scanned.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of the prior art.
FIG. 2 is a schematic diagram of a first embodiment of the scanning
system of the present invention.
FIG. 3 is a schematic diagram of the lens and CCD imager of the
scanning system of FIG. 2.
FIG. 4A is a schematic diagram of an optical path selection device
used in the scanning system of FIG. 2.
FIG. 4B is a schematic diagram of a preferred object plane
placement of the optical paths of the scanning system of FIG.
2.
FIG. 5A is a schematic diagram of a linear CCD used in the scanning
system of FIG. 2.
FIG. 5B is a schematic diagram of an alternative embodiment of the
selection device shown in FIG. 3.
FIG. 6 is a schematic diagram of a second embodiment of the
scanning system of the present invention.
FIG. 7 is a schematic diagram of a third embodiment of the scanning
system of the present invention.
FIG. 8 is a schematic diagram of a fourth embodiment of the
scanning system of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
A stationary scanning apparatus 8 according to the present
invention is shown schematically in FIG. 2. The scanning apparatus
8 includes a light source 10, and an optical system having a
redirecting mirror 11, a fixed focal length lens system 12, a first
mirror 13, a second mirror 14, and a third mirror 15. The apparatus
also uses a light sensing device such as a linear CCD imager 16
which is positioned in an image plane 17. The CCD imager 16, which
is shown in FIG. 5A, is coupled to a traveling nut 18 which
threadably engages a vertically oriented threaded shaft 19. The
shaft 19 is rotationally driven by a motor 20. The nut 18 is also
connected to a slider 21, which slides along a slider guide 22,
causing the CCD imager 16 to traverse the image plane 17 when the
motor 20 is driven.
Objects, such as boxes 50 having an upper surface 52 containing a
bar code (not shown), are scanned as they move along on a conveyor
belt 23 past the scanning apparatus 8. The boxes 50 being scanned
may vary in size from some predetermined minimum accommodatible
object size, shown by box 50a to some predetermined maximum
accommodatible object size, shown by box 50c. An intermediate size
box is shown as box 50b. The light source 10 is aligned to
illuminate the upper surface 52 of each size box. The first mirror
13 lies along an optical path that is aligned with a scanning image
on box 50c having the maximum accommodatible size, the third mirror
15 lies along an optical path that is aligned with a scanning image
on box 50a having the minimum accommodatible object size, and the
second mirror 14 lies along an optical path that is aligned with a
scanning image on box 50b. Relative to some reference height, such
as the conveyor belt, the difference between the length of the
optical path containing the first mirror 13 and the length of the
optical path containing the second mirror 14 is equal to the depth
of field of the CCD imager 16 and the lens system 12. This is also
true for the difference between the length of the optical path
containing the second mirror 14 and the length of the optical path
containing the third mirror 15. The depth of field of a CCD imager
and a lens system can be calculated approximately by the
equation:
where f/# is the f-number of the lens system 12, PP is the pixel
pitch for the CCD imager 16, and M is the object magnification of
the optical system.
In the present embodiment there are three distinct optical paths
upon which light reflected from the bar code may travel. First, the
light may travel along the optical path containing the second
mirror 14. When this path is used, the light reflected by the
second mirror 14 travels along the optical axis of lens system 12.
The light reflected from the bar code may also travel along either
of the respective optical paths containing the first and third
mirrors 13 and 15. These mirrors do not reflect the light directly
along the optical axis of lens system 12 since they are positioned
away from the optical axis of lens system 12. Here, the first
mirror 13 is positioned below the optical axis and the third mirror
15 is positioned above the optical axis. By positioning the first
and third mirrors 13 and 15 in this manner, the optical path of the
light reflected from each mirror forms a unique predetermined angle
with the optical axis of lens system 12. Further, the positioning
of mirrors 13, 14, and 15 relative to the optical axis of lens
system 12 causes the light traveling along the optical path of each
mirror to fall on the same image plane 17 such that each of the
optical paths has an independent field of view. The optical path
corresponding to each of the mirrors 13, 14, and 15, can be
individually selected for reception by powering the motor 20 to
cause the CCD imager 16 to traverse the image plane 17 until a
general position sensing device 24, such as an LVDT or linear
decoder, determines that the CCD imager 16 is located at the
position in the field of view that corresponds to the selected
optical path.
An enlarged view of the fixed lens of the lens system 12 and the
CCD imager 16 used in the scanning apparatus 8 is shown in FIG. 3.
FIG. 3 illustrates the CCD imager 16 selectively traversing the
image plane 17 from a position 16c to a position 16a in a direction
perpendicular to a scanning direction to change the selected field
of view from the optical path corresponding to the third mirror 15
to the optical path corresponding to the first mirror 13. This
movement of the CCD imager 16 is produced by selectively powering
the motor 20 to move the nut 18 upward on the shaft 19 a
predetermined distance with, for example, a step motor, or
preferably until a general position sensing device 24 determines
that the CCD imager 16 is positioned to the appropriate location
within the field of view corresponding to the path of the first
mirror 13.
Initially, as one of the boxes 50 moves along the conveyor belt 23,
its height is determined by a height detector 25 shown in FIG. 2. A
light emitter 26 emits light which is detected by at least one of a
plurality of light sensors 27 of the height detector 25, as also
shown in FIG. 2. A signal that is proportional to the height of the
detected one of the boxes 50 is produced from the plurality of
light sensors 27 of the height detector 25. Alternatively, the
height detector 25 and the scanning apparatus 8 could be positioned
in a manner (i.e., 90.degree. from its depicted direction) to
determine the width or size of the box 50.
The signal from the height detector 25 is processed by logic
circuitry 32, shown in FIG. 4A, which selects one of the three
optical paths containing either first mirror 13, second mirror 14,
or third mirror 15. The logic circuitry by analyzing the signal
from the height detector determines if the box 50 falls within one
of three ranges, small, medium, and large. If the logic circuitry
determines that box 50 falls within the small range (i.e., it is
around the minimum accommodatible height of box 50a), then it sends
a signal, which is conditioned by motor driving circuit 33, to
drive motor 20 moving the CCD imager 16 along the image plane 17 to
the field of view of the optical path containing the third mirror
15 with the aid of the position sensing device 24. By this same
process, if the logic circuitry 32 determines that the box 50 falls
within the medium range (i.e., around the height of box 50b), then
it will select the optical path containing the second mirror 14,
and if it determines that the box 50 falls within the large range
(i.e., around the height of box 50c), then the logic circuitry 32
will select the optical path containing the first mirror 13. The
logic circuitry 32, as stated above, selects an optical path by
sending a signal to a motor driving circuit 33 which drives the
motor 20. The motor 20 is driven in a designated rotational
direction to cause the threaded shaft 19 to displace the CCD imager
16 in the image plane 17 in a direction perpendicular to the scan
direction until the position sensing device 24 indicates that the
CCD imager 16 is aligned with the field of view corresponding to
the optical path of the mirror 13, 14, or 15 selected by the logic
circuitry 32.
By selecting the optical path in the manner stated above, the total
optical path length that the light reflected from the scanning
image on the box must travel to reach the CCD imager 16 can be
maintained within a certain predetermined range. This predetermined
range is selected so that the total change in optical path length
stays within the depth of field of the lens system 12 and the CCD
imager 16, thereby eliminating the need to focus the lens system
12. The predetermined range is also selected so that the scanning
apparatus 8 will stay within the amount of overscan that can be
tolerated by the CCD imager 16.
As shown in FIG. 2, the distance that light must travel from the
redirecting mirror 11 to the lens system 12 varies incrementally
for each of the three optical paths which each contain one of the
mirrors 13, 14, or 15. This incremental difference represents a
range in which the overscan and the depth of field of the lens
system 12 and the CCD imager 16 can be tolerated. An optical path
is selected corresponding to a detected object height so that the
distance from the scanning image on the object to the image plane
17 can be held within the range in which the lens system 12 and CCD
imager 16 can operate without requiring the lens system 12 to
change focal length or re-focus. In this manner, the scanning
apparatus 8 compensates for different object heights in discrete
steps, and the number of these discrete steps, or optical paths
available to maintain the predetermined range between the CCD
imager 16 and the scanning image, will depend on the number of
mirrors used. The illustrated embodiment shows use of three mirrors
13, 14, and 15, but additional or fewer mirrors may be used to
accommodate the number of object heights to be scanned as will be
described below.
The number of optical path length compensating mirrors required by
a scanning system depends primarily on the depth of field of the
fixed lens system used as well as the overscan that can be
tolerated. For example, if a 20 mil code is to be scanned over a
box height range of 36 inches with an f/8 lens, a minimum of six
optical path compensating mirrors would be required if the depth of
field for such a system is approximately six inches. A scanning
apparatus designed in this manner would have a minimum of overscan
and thus would require less pixels for scanning labels at different
distances, thereby allowing an increase in the speed of the
conveyor belt 23.
The number of mirrors, such as folding mirrors, necessary to
provide an optical path length within the predetermined range for
objects varying from the predetermined minimum accommodatible
object size to the predetermined maximum accommodatible object size
can be minimized. For example, if the optical system for the
optical path of each of the mirrors 13, 14, and 15 has a respective
object plane 80, 81, and 82 set to a different height above some
reference, such as the conveyor belt, as shown in FIG. 4B, then
every scannable box height must either lie on one of the object
planes or within the depth of field above and below them. The
optical path used to read the scanning image on an object having a
height falling within the small range (i.e., around box height 50a)
should have an object plane 80 that is above the conveyor belt 23,
by an amount equal to the predetermined minimum accommodatible
object size 84 plus half the depth of field 83 of both the lens
system 12 and the CCD imager 16. Further, the optical path used to
read the scanning image on an object falling within the large range
(i.e., around box height 50c) should have an object plane 82 that
is above the conveyor belt 23 by an amount equal to the maximum
accommodatible object size 85 less half the depth of field 83 of
the lens system 12 and the CCD imager 16, as shown in FIG. 4B. The
optical path used to read objects falling within the medium range
(i.e., around box height 50b) should have an object plane 81
located equidistant from the object planes 80 and 82. In an
arrangement such as this, once the object height has been
determined by the detector 25, the optical path having an object
plane closest in height to the object is selected by the logic
circuitry 32, shown in FIG. 4A.
FIG. 5A is a cross-sectional diagram of the linear CCD imager 16.
An unused portion 61 and a used portion 62 of the CCD imager 16 are
illustrated. The length of the used portion 62 of the CCD imager 16
is based on the minimum number of pixels necessary to scan the
largest box size.
FIG. 5B is a schematic cross-sectional diagram of an alternative
embodiment of the selection device of FIG. 4A. In this alternative
embodiment, the CCD imager 16 is moved in a direction substantially
perpendicular to the scan direction and substantially parallel to
the image plane 17. The CCD imager 16 is coupled to a shaft 40
which pivots about a flexure 41 such as Lucas Aerospace's
"FREE-FLEX" pivot. It is important that the shaft 40 be
sufficiently long so that when it pivots about the flexure 41, the
CCD imager 16 will travel substantially parallel to the image plane
17. The pivoting movement is caused by a driving force applied
perpendicular to the shaft 40 by either a solenoid or a linear
motor 42, which is controlled by the height detector 25, the
position sensing device 24, and the logic circuitry 32, as
previously described for the first embodiment and depicted in FIGS.
2 and 4A.
FIG. 6 is a cross-sectional schematic diagram of a second
embodiment of the present invention. In this embodiment, many of
the elements are common to the first embodiment, as depicted in
FIGS. 2 and 4A. For example, objects of varying height are conveyed
along a conveyor belt 23 and light traveling along a designated
optical path is received by a CCD imager 16 after first passing
through a lens system 12. However, in this embodiment, the CCD
imager 16 is fixed at a position normal to the optical axis of the
lens system 12, and the selection process is performed in object
space instead of image space. Further, the light reflected from a
bar code (not shown) is directly received by the first, second or
third mirror 13, 14 or 15 depending on the height of the object. An
optical path containing one of the mirrors 13, 14, or 15 is
selected by a pivoting mirror 61, which is pivoted or rotated to
reflect light from a selected field of view corresponding to the
selected optical path along the optical axis of the lens system 12
and onto the CCD imager 16. The position of the pivoting mirror 61
is determined by signals from the height detector 25 and the
position sensing device 24, as described for the first embodiment
shown in FIGS. 2, 4A, and 4B. The actual pivoting of the pivoting
mirror 61 can be performed by a mechanism similar to that used to
pivot the CCD imager 16 in the alternative embodiment to the
selection device shown in FIG. 5B. However, in this embodiment, the
length of the shaft 40 should be nominal to ensure that the surface
63 of the pivoting mirror 61 is capable of changing its angle with
respect to each of the mirrors 13, 14, and 15.
FIG. 7 is a cross-sectional schematic diagram of a third embodiment
of the present invention. This embodiment is similar to the
embodiment in FIG. 6 except that the selection of the optical path
corresponding to either mirrors 13, 14 or 15 is determined by a
rotating diffractive or a refractive element 71. These elements can
be wavelength dependent and can be produced by a holographic
process.
In the third embodiment shown in FIG. 7, the rotating element 71
selects an optical path corresponding to one of the mirrors by
rotating so that only a field of view that corresponds to the
selected path will be refracted onto the CCD imager 16. The
diffractive or refractive element 71 is rotated so that it will
direct only one of the optical paths from the first, second or
third mirror 13, 14 or 15 through the lens system 12 and onto the
CCD imager 16. As in the previous embodiments, an optical path
corresponding to one of the mirrors is selected for a range of
object distances so that the total optical path length from the CCD
imager 16 to the object being scanned remains within a constant
range.
FIG. 8 is a cross-sectional schematic diagram of a fourth
embodiment of the present invention. In this embodiment, many of
the elements are common to the third embodiment shown in FIG. 7.
For example, objects of varying distance from a CCD imager 16 are
scanned, and light traveling along a designated optical path is
received by the CCD imager 16 after first passing through a lens
system 12. In this embodiment, the CCD imager 16 is fixed at a
position normal to the optical axis of the lens system 12. Further,
the light reflected from the bar code (not shown) is directly
received by the first, second, or third mirror 13, 14, or 15
depending on the height of the object. An optical path containing
one of the mirrors 13, 14, or 15 is selected by an acousto-optic
modulator 90, which is positioned in the image plane between the
lens system 12 and the CCD imager 16. The acousto-optic modulator
90 uses a low voltage RF signal to launch an acoustic wave into a
block of fused silica, where the wave sets up a sinusoidal
refractive index grating which scatters the incident beams out of
their original direction with good efficiency. The switching speed
of the modulator 90 is usually no more than a few microseconds
allowing it to be highly responsive to a signal from the height
detector 25, which selects the optical path that will provide a
predetermined range of distances between the CCD imager 16 and the
object being scanned. Since the acousto-optic modulator 90 is
designed to switch monochromatic light, a monochromatic filter 92
is mounted onto the object side of the lens system 12.
By maintaining a constant optical path length range from an object
to the CCD imager 16, the previously discussed embodiments allow a
scanning optical apparatus to incorporate a fixed focus non-zoom
lens within the scanning system, thereby creating a more reliable,
less expensive and longer lasting lens system within the optical
scanning apparatus.
Except as otherwise disclosed herein, the various components shown
in outline or block form are individually well-known and their
internal construction and their operation is not critical either to
the making or the using of this invention or to a description of
the best mode of the invention.
While the detailed description above has been expressed in terms of
specific examples, those skilled in the art will appreciate that
many other configurations could be used to accomplish the purpose
of the disclosed inventive apparatus. Accordingly, it will be
appreciated that various modifications of the above-described
embodiments may be made without departing from the spirit and scope
of the invention. Therefore, the invention is to be limited only by
the following claims.
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